Signal and noise separation utilizing zero crossing electron tube and circuit

ABSTRACT

The invention consist of an electron tube and circuitry which operate to determine whether a zero crossing of a wave consisting of signal mixed with noise is followed by a second zero crossing at a time not less than a predetermined interval. Accordingly, an output signal is produced whenever a true signal is present at the input; thereby, separating signals from noise.

United States Patent Zinn [451 July 18,1972

[54] SIGNAL AND NOISE SEPARATION UTILIZING ZERO CROSSING ELECTRON TUBE AND CIRCUIT [72] Inventor: Mortimer l1. Zinn, West Long Branch,

[73] Assignee: The United States of America as represented by the Secretary of the Army [22] Filed: Aug. 26, 1964 [21] App1.No.: 398,139

Related U.S. Application Data [62] Division of Ser. No. 144,799, Oct. 12, 1961.

[52] U.S. Cl. ..313/78, 250/491 R, 313/80,

315/3 [51] Int. Cl. ..l-I0lj 29/74 [58] field of Search ..313/293, 303, 78, 80;

Primary ExaminerCarl D. Quarforth Assistant Examiner-J. M. Potenza Attarney-Harry M. Saragovitz and Julian C. Keppler ABSTRACT The invention consist of an electron tube and circuitry which operate to determine whether a zero crossing of a wave consisting of signal mixed with noise is followed by a second zero crossing at a time not less than a predetermined interval. Accordingly, an output signal is produced whenever a true signal is present at the input; thereby, separating signals from noise.

1 Claim, 7 Drawing figures UNIFORM MAGNETIC FIELD OVER REGION YOF CURVED DEFLECTION PLATES Pmmmuuemz 3.678.321

SHEET 1 BF 2 FIG I FIGZ FIG.3

I N VEN TOR MO.RTIM R H. ZINN ATTORNE\ PATENTED .mu 8 I972 SHEET 2 [IF 2 UNIFORM MAGNETIC FIELD OVER REGION FIG,4

OF CURVED OEFLECTION PLATES LEVEL REFE RENCE I T I I I INPUT TO IST DEE PLATES OUTPUT IIWIIJIJIIIIII FIG.6

' INPUT TO I ST DEF. PLATES OUTPUT FIG.7

I N O T O R 8 l E P "w E U D M l W C v A R T m f E C I 2 R L 0 m R MN m I T M /4 VF. I 2 L D m FP 3 M -A film R Lm N AM u ML M L m S E L lI r p M R A R L E T II n M S N R A E R X T E L P U D IN VEN TOR, MORTIMER H. ZINN M 14 ATTOR N EY SIGNAL AND NOISE SEPARATION UTILIZING ZERO CROSSING ELECTRON TUBE AND CIRCUIT This application is a division of application Ser. 144,799, filed Oct. 12, 1961.

This invention relates to a method and apparatus for separating signals from noise, for statistical wave analysis, and the like. More specifically, the invention relates to a novel cathode-ray tube system in which recurring signals are detected by plural sets of deflecting plates spaced axially along the path of the electron beam of the tube.

The invention consists of an electron tube and circuit which operate to detemiine whether a zero" crossing of a wave consisting of signal mixed with noise is followed by a second zero" crossing at a time not less than a predetermined interval. The tube itself with its associated circuit can, therefore, be used to provide an output signal whenever a true signal is present at the input. Since a single tube circuit is used to accomplish this job compared to the complex circuits now required, this means of distinguishing between signal and noise can be added to almost any pulse signal transmission system, either of the radar or communication type, at low cost and with little circuit design effort.

The zero crossing tube, for which the name Zeecistron is coined, utilizes the inherent versatility of an electron beam to accomplish a task which would otherwise require a multiplicity of tubes and circuits. In the preferred embodiment an accelerating electrode is used to produce an electron beam which is acted upon by the input signal (a mixture of true signal and noise) on a pair of first deflection plates. The result of this action is to sweep the electron beam through an excursion on an apertured electrode with the beam crossing the aperture every time a zero crossing of the signal-noise mixture has occurred. Electrons falling on the aperture pass through it into the second region of the tube at every zero crossing whether it is a positive going crossing or a negative going crossing and whether the crossing was due to noise or a signal. The electrons which pass the aperture now enter a second deflection plate system which consists of two plates, spaced equally over their length, but folded or curved to obtain a specifically developed length. This deflection system is located in a uniform magnetic field which is perpendicular to the electric field developed by the voltage across the two plates and is also perpendicular to the direction of motion of the electrons entering the deflection plate system. The voltage across the plates is derived by DC. coupling to a resistor in series with the apertured electrode so that when current is flowing to the apertured electrode the voltage is sufficient to produce a force on electrons in the deflection plate system which nearly balances the force produced by the magnetic field. Thus electrons which have passed through the aperture will continue to flow, at the velocity of entry into the deflection plate system, along an equipotential line between the two deflection plates as long as the electron beam in the first region remains on the apertured electrode. If a second zero" crossing, however, occurs before the electrons due to the first crossing have passed through the second deflection plate system, the voltage across the second deflection plates is reduced toward zero as the current to the apertured electrode drops. This unbalances the foreces acting on the first electrons and causes them to be collected by one of the deflection plates. No signal appears at the output electrode in this case. Since the time of transits through the deflection plate system can be adjusted by adjusting the velocity of the electrons entering the second deflection plate system the tube has performed the function of generating an output signal as a result of a zero" crossing which has not been followed by a second crossing in less than a predetermined time interval. Additional details. and other methods of operation and uses for the Zeecistron tube are contained in the detailed description given below.

It is therefore a primary object of the invention to provide a method and apparatus for separating recurring signals from house.

It is a further object of this invention to provide a method and apparatus for producing an output signal when input signals have a specific relationship in time.

It is an additional object of this invention to provide an electron beam device for achieving the above and other objects.

It is another object of the invention to provide a circuit utilizing the electron beam device for detecting signals in the presence of noise, and the like.

Other objects and features of the invention will be apparent in the following detailed description taken in connection with these drawings in which corresponding elements are represented by the same reference characters throughout:

FIG. 1 represents the basic zero-crossing tube structure;

FIG. 2 is an embodiment of the zero-crossing tube of FIG. 1 in a typical circuit;

FIG. 3 represents an embodiment of the zero-crossing tube structure using curved deflection plates;

FIG. 4 is a circuit using the zero-crossing tube of FIG. 3;

FIG. 5 and 6 show characteristic waveforms of signals which may be impressed on the first deflection plate of zero-crossing detector tube and waveforms of the respective output signals derived from the output electrode of the tube; and

FIG. 7 shows the invention in conjunction with a radar system.

The electron beam is an extremely versatile device. It can be used to perfon'n functions which would require a multiplici ty of devices and circuits to accomplish the same task. An example of this is the Zero-Crossing System of detection which can be accomplished with a complex circuit containing multivibrators, difierentiators, rectifiers, adders, phaseinverters and sawtooth generators. All of these devices can be replaced by an electron tube of special design operating in an appropriate circuit.

First, the Zero-Crossing System will be examined to determine'its characteristics. The Zero Crossing System of detection is based on the principle that the average time interval between successive zeros of random noise is small compared to the duration of a pulse to be detected. A successful zero crossing detector should, therefore, generate a pulse each time a zero-crossing (or crossing of any predetermined potential level) occurs and reject all signals which are followed by another signal in less than a predetermined time. An electron beam device capable of performing these functions will now be described.

The device shown in FIG. 1 is a type of cathode-ray tube having heater 1, a cathode coating 3 on cathode 2, an accelerating electrode 4, a first pair of deflection plates 5 followed by an apertured electrode 6, a second pair of deflection plates 7, and an output electrode or anode 8. It is to be noted that deflection plates 7 are considerably longer than plates 5. For zero crossing system operation, the tube is incorporated in a circuit as shown in FIG. 2. The input voltage E in consisting of a mixture of noise and the signals to be detected is applied to the first pair of deflection plates 5. This voltage typically will be the video output of a radar system.

When a voltage exists between these deflection plates the electron beam is intercepted by apertured electrode 6. As the voltage changes from positive to negative, for example, (or crosses a set potential level established by a bias between the two deflection plates), the beam is swept across the aperture in electrode 6, permitting electrons to penetrate the space beyond the apertured electrode. Thus electrons pass the aperture every time a zero" crossing occurs. The first set of deflection plates and the apertured electrode, described so far, act as a built-in amplifier to provide an amplified signal to the second set of plates when a zero crossing occurs.

The potential on the second set of deflection plates 7 are adjusted so that there is no voltage between them whenever the beam is intercepted by the apertured electrode. This is accomplished by adjusting the voltage on plate 7 by means of potentiometer, P to be equal to that due to the plate supply voltage Ebb less the drop across resistor R due to the flow of intercepted beam current, which has been applied to opposite plate 7" by direct coupling to apertured electrode 6. Electrons which have passed the apertured electrode during a zero crossing will see no deflection potential as long as the electron beam passing through the first set of deflection plates continues to fall on the apertured electrode.

A second crossing of the aperture, however, will reduce the beam current to electrode 6, resulting in the appearance of a potential between the second pair of deflection plates, since the decrease in collected beam current will reduce the drop across resistor R raising the potential of deflection plate 7" directly coupled to the apertured electrode as compared to the opposite plate 7' whose potential is fixed by potentiometer p If this second crossing of the aperture has occurred before the electrons flowing between the second pair of deflection plates 7 due to the first cross-over have passed beyond a point where the application of the deflection voltage will cause the electrons to be deflected sufficiently to strike the deflection plate, no current will flow in the output circuit. If, however, the beam has remained on the apertured electrode for a sufficiently long period of time without a zero" crossover, the electrons due to the first crossover will travel between the second deflection plates without deflection and will be collected at the output electrode. This causes an output pulse through output coupling capacitor C or equivalent A.C. coupling device. The basic design shown in FIG. 2 will give a pulse output whenever a zero crossing has not occurred during a time period:

where L is the length of the second deflection plate and E,,,, is the voltage of the aperture electrode, which determines the velocity of the electrons passing through the aperture.

If a crossover occurs before time t has elapsed the electrons will be captured by one of the plates. The capture of the electrons by the deflection plates will also depend on the separation of the deflection plates.

Equation 1 indicates that in order to operate in a time region which will be of interest, the length of the second deflection plates will have to be long and the accelerating potential of the apertured electrode will have to be low. The overall length of the second deflection plates, which afiects the physical size of the tube, may be kept within reasonable bounds by using another embodiment of the invention as shown in FIG. 3 when operated in a circuit such as that shown in FIG. 4.

The device shown in FIG. 3 is constructed in a manner similar to that of FIG. 1 up to the apertured electrode. The second deflection plates 9 now consist of a curved pair of plats 9' and 9" with a constant separation over the length of the plates. Electrons which pass through apertured electrode 6 and enter the curved deflection plate system with a velocity V, will flow through the deflection plate system and be collected by the output electrode 8 provided that the curved deflection system is in a magnetic filed B which tends to balance out the voltage across the deflection plates, Vd, in accordance with the relationship: B Vd/a v where a is the separation between the deflection plates.

Referring now to FIG. 4, the curved version of the zero crossing detector will operate in the following manner. Electrons are accelerated by accelerating electrode 4 through first deflection plates where the input signal E, deflects the beam. At the time of a zero" crossing, electrons pass through the aperture of the apertured electrode. The setting of potentiometer P has been adjusted to derive a voltage Vd across the deflection plates which produces a force on the electrons sufficient to nearly, but not completely, cancel the force produced by the constant magnetic field B which is perpendicular to the direction of the initial velocity of the electrons as well as the deflection voltage. As long as the apertured electrode is collecting full current to maintain Vd by the voltage drop across P this approximately balance will be maintained and the electrons will follow an equipotential line until they arrive at the output electrode. If, however, a second "zero crossing occurs before the electrons have passed through or almost through the deflection system, the voltage across the deflection plates will drop from Vd towards zero. This removes the electric field which has maintained the nearbalance with the magnetic field, and the electrons within the deflection plate system will be deflected towards the deflection plate which was previously the negative plate of the two deflection plates, traveling in a circular path with a radius R M v /e B. If the separation between the electrodes, a, has been selected such that a is less than 4R (or, conversely, for a fixed separation between the plates, v has been adjusted so that R a/4), the electrons will be collected by the deflection plate before a half circle has been completed. The curved version of the zero crossing detector tube will, therefore, provide an output signal only when a second "zero crossing has not occurred.

Other methods of extending the time delay in the second system (FIG. 4) may also be used. One such possibility is the use of a magnetic field value which will yield cycloidal or trochoidal motion within the deflection plate system until the motion is disturbed by a second zero crossing of the aperture.

The relationship between the input signal to the first set of deflection plates is shown for several illustrative cases in FIGS. 5 and 6. As can be seen, a signal appears at the output after a zero crossing has occurred at the input provided that a second crossing has not occurred before the electrons, which have entered the second deflection plate system as a result of the first crossover, are collected by the anode. The time interval td is a function of the developed linear length of path between the curved, second deflection plate system and is inversely proportional to the velocity of the electrons, or the square root of the aperture electrode voltage. This time interval can, therefore, be adjusted over a considerable range, to achieve time delay levels as large as 5 microseconds or greater. The output pulse is delayed with respect to the leading edge of an input pulse (first crossover) by a duration equal to time time of flight of electrons in the second set of deflection plates. Thus the device can be used as an electronic variable delay line by inserting a pulse whose duration is equal to or grater than the maximum achievable time delay and by varying the aperture voltage and the second deflection plate voltage. lncreasing these two voltages while maintaining the relationship (assuming the curved form of second deflection plates) I Vd Eap where Vd the voltage across the deflection plates in the absence of a zero crossing and Eap =the aperture voltage will result in a decreasing time delay. This type of operation is illustrated in FIG. 5, with the time delay set for a maximum.

FIG. 6 illustrates the rejection and acceptance characteristics of the device; it does not necessarily correspond to a useful type of operation. As can be seen in the Figure, input pulses greater than a set value of time delay result in an output pulse delayed with respect to the start of the pulse. No pulse output appears because of the end of the pulse since the spacing between pulses is less than the critical value of delay Id.

The tube and circuits of FIGS. 2 and 4 may be used in a radar system by coupling the zero-crossing tube to the video output of the receiver, as shown in FIG. 7, in which the zerocrossing detector system 21 is inserted between video amplifier 22 of radar receiver 23 and indicator 24. The video information is applied to the first set of deflection plates as the input signal to the tube 21. An output pulse is obtained only when the time between a pair of zero crossings is greater than the transit time of electrons within the second deflection plate system. Since there is a statistically significant difference in the time between crossings due to noise and the time between crossings due to a pulse of known width, the probability of detecting the presence of a known pulse can be made to be high by proper choice of the critical time delay within the second set of deflection plates, with a small probability of having an output pulse due to noise alone. The zero-crossing tube, therefore, serves to separate the noise from the pulse and to provide a pulse output with a fixed delay (for which compensation can be made in the radar range circuit) and with a fixed pulse width and height. With proper design of the tube and choice of circuit constants, the output amplitude can be sufficient to be fed directly to a cathode-ray tube for visual presentation. (Essentially the zero-crossing tube can be designed to provide voltage amplification as well as perform the function of eliminating the noise signals.) If it is desired to process the data further, such as for MT! purposes, etc., and it is desired to reconstitute the original pulse signal, the output of the zero-crossing tube can be used to trigger a 'rnonostable multivibrator or the equivalent to generate a pulse whose duration is equal to the original duration of the transmitted pulse.

The concept of the tube and cooperating circuitry was generated primarily as a means of detecting pulses hidden by natural or man-made noise. The method by which this is accomplished is described above and in the discussion of use in a radar system. The tube and circuit could be used in a secrecy system for either radar or pulse-code communication where the original pulse signals are mixed with video noise and the combined resultant is used to modulate a CW amplifier. in ad dition, since the tube is basically a correlation device; it could be used in other types of cross-correlation circuits.

The device can be used as well in the important field o statistical analysis of waveforms in radar and communication. Present methods employ a multiplicity of electronic devices whereas the device disclosed herein carries on all operations in one tube. The result is that measurements may be made more simply and quickly and probably with greater accuracy.

The foregoing disclosure relates to a preferred embodiment of the invention. Numerous modifications or alterations may be made therein without departing from the spirit and scope of the invention set forth in the appended claims.

What is claimed is:

1. An electron tube comprising: an electron gun for initiating a beam of electrons; an accelerating electrode spaced beyond said electron gun in the direction of travel of said electron beam; a first pair of parallel deflection plates spaced beyond said accelerating electrode and axially aligned with the axis of said electron beam; an apertured electrode spaced beyond said first pair of parallel deflection plates, the axis of the aperture of said apertured electrode being coaxial with the axis of said electron beam; a second pair of continuous parallel deflection plates spaced beyond said apertured electrode in the direction of travel of said beam, said second pair of plates being considerably longer than said first pair of plates and being so curved that said electron beam when traveling between said second pair of plates undergoes at least two changes in direction; and an anode placed beyond said second pair of deflection plates in the direction of travel of said electron beam. 

1. An electron tube comprising: an electron gun for initiating a beam of electrons; an accelerating electrode spaced beyond said electron gun in the direction of travel of said electron beam; a first pair of parallel deflection plates spaced beyond said accelerating electrode and axially aligned with the axis of said electron beam; an apertured electrode spaced beyond said first pair of parallel deflection plates, the axis of the aperture of said apertured electrode being coaxial with the axis of said electron beam; a second pair of continuous parallel deflection plates spaced beyond said apertured electrode in the direction of travel of said beam, said second pair of plates being considerably longer than said first pair of plates and being so curved that said electron beam when traveling between said ''''second pair of plates undergoes'''' at least two 180* changes in direction; and an anode placed beyond said second pair of deflection plates in the direction of travel of said electron beam. 